Methods for fat signal suppression in magnetic resonance imaging

ABSTRACT

The present invention is directed to methods for chemical species signal suppression in magnetic resonance imaging procedures, wherein Dixon techniques are enhanced by continuously sampling techniques. In the invention, k-space data is acquired during the entire period of read gradient associated with a gradient echo pulse acquisition scheme. The invention utilizes a total sampling time (TST) acquisition during the entire read gradient, using three echoes of a TST data set to achieve chemical species separation in both homogenous fields as well as areas of field inhomogeneity. As an example, a continuously sampled rectilinearly FLASH pulse sequence is modified such that the time between echoes was configured to be 2.2 milliseconds, with TE selected to allow 180° phase variation in the fat magnetization between each of the three TE&#39;s (TE 1 , TE 2 , and TE 3 ). Data collected during the dephase and rephase gradient lobes are defined as a first Dixon acquisition, with data collected by the read gradient lobe being defined as a second Dixon acquisition. Two point Dixon reconstruction techniques are used to form images for each chemical species, such as for generating water and fat images of the scanned object region. Other corrections, such as off-resonance correction may be applied on the image data.

CROSS-REFERENCE TO RELATED APPLICATIONS

This U.S. Continuation patent application claims priority to and thebenefit of U.S. patent application Ser. No. 12/044,442 filed Mar. 7,2008, which claims priority to and the benefit of U.S. ProvisionalPatent Application Ser. No. 60/894,070 filed Mar. 9, 2007, which isincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a method of fat signal suppression orseparation of fat and water signals in MRI imaging of a scanned objectregion using rectilinear Dixon techniques.

BACKGROUND OF THE INVENTION

In MRI imaging of chemical species such as fat and water within bodytissue, the misregistration of fat in acquired magnetic resonance imagescan seriously degrade the quality of the image and obscure importantpathology. As a diagnostic tool, such off-resonance effects, which mayarise from field inhomogeneities, susceptibility or chemical shift,cause artifacts in the acquired images. The artifacts appear aspositional shifts along the read direction in rectilinearly sampledacquisitions. Although various fat suppression methods have beenconceived, such as using specialized radio-frequency (RF) excitationpulse sequences, such methods have various limitations. For example,inversion recovery pulse sequences and chemical shift selectiveexcitation pulses have been attempted, but are limited by extendedacquisition times and specific absorption rate (SAR) constraints whenused as a diagnostic tool for human patients.

There have been various attempts at reducing scan times in obtainingreconstructed images of water-fat decomposition, but there is a furtherneed for short scan times while obtaining high quality images. Dixontechniques have been utilized for water-fat decomposition in rectilinearsampling schemes. In a Dixon technique, water and fat images aregenerated by either addition or subtraction of the “in-phase” and“out-of-phase” data sets. With magnetic field inhomogeneity, suchtechniques may still lead to inaccurate decomposition of water and fatsignals, resulting in a blurred image. In a two-point Dixon (2PD)technique, the two acquired images can be contaminated by off-resonanceeffects such that the water and fat images have significant componentsfrom the unwanted chemical species.

Modified Dixon techniques, such as the three-point Dixon (3PD) techniquehave also been developed to correct for magnetic field inhomogeneityoff-resonance effects as well as susceptibility effects. The advantageof these multiple-point Dixon techniques over specialized RF excitationpulse-sequences or pulses lies in, the ability to acquire water-fatseparation even in the presence of magnetic field inhomogeneities, suchas tissue-induced local inhomogeneity or applied magnetic fieldinhomogeneities in the field of view.

Although the multi-point Dixon techniques avoid SAR limitations, thetypical Dixon implementation requires two acquisitions with differentTE's to create a frequency map for off-resonance corrections. As anexample, two acquisitions with different TE's are utilized where theΔTE=2.2 MSEC @ 1.5 T. This characteristic ΔTE generates two complex datasets with 180° relative phase variation in the fat magnetization. Thelong acquisition times required for two separate acquisitions impose asignificant limitation on the use of such techniques. It would thereforebe desirable to increase the efficiency of the Dixon technique to reduceacquisition time required for accurate fat-water separation.

SUMMARY OF THE INVENTION

The present invention is therefore directed to methods for fat signalsuppression in magnetic resonance imaging procedures, wherein Dixontechniques are enhanced by continuously sampling during the entireperiod of read gradient. The invention utilizes a total sampling time(TST) acquisition during the entire read gradient, using three echoes ofa TST data set to achieve fat-water separation in both homogenous fieldsas well as areas of field inhomogeneity. In an embodiment, acontinuously sampled rectilinearly FLASH pulse sequence is modified suchthat the time between echoes was configured to be 2.2 milliseconds, withTE selected to allow 180° phase variation in the fat magnetizationbetween each of the three TE's (TE1, TE2, and TE3). Any gradient echosequence (FLASH, true-FISP, FISP, or other gradient echo sequences) arecontemplated. Data collected during the dephase and rephase gradientlobes are defined as a first Dixon acquisition, with data collected bythe read gradient lobe being defined as a second Dixon acquisition.Water and fat images are obtained according to the invention, andoff-resonance correction is applied if desired.

More particularly, the systems and methods according to the presentinvention, in an embodiment, provide for constructing an image from anMRI data set, wherein the MRI data is acquired with a total samplingtime (TST) acquisition, with a predetermined timing relationship betweenechoes during a single acquisition process. Accordingly, the presentinvention provides significant improvement over traditional 2-pointDixon methods in terms of time savings in data acquisition, whileproviding desired suppression of fat or water signals. The totalacquisition time is significantly reduced as compared to traditional2-point Dixon methods in comparable sequences. These objectives may berealized using an apparatus to produce MRI images of a scanned objectregion with suppression of fat signals to produce high contrast waterand fat images of the scanned object region. The apparatus may comprisea MRI scanner for obtaining images of a predetermined volume of ascanned object region, and a controller configured to control operationof the scanner to acquire images using a modified Dixon techniquewherein data acquisition is performed over substantially the entire readgradient of a gradient echo pulse sequence performed on the scanner. Thepulse sequence produces data acquired to form a plurality of magneticresonance imaging data signals which are processed using Dixonreconstruction techniques to create water and fat images of the scannedobject region. An output device for display of the formed images.

The invention is also directed to a method for producing chemicalspecies images, such as water/fat images, of a scanned object regionwith suppression of the respective water and fat signals in alternativeimages. The method comprises the steps of acquiring k-space data oversubstantially the entire read gradient of a gradient echo pulse sequencehaving a dephase lobe, a readout lobe and a rephase lobe. The methodcombines k-space data from the dephase and rephase lobes of the readgradient to form a first Dixon acquisition and the data acquired fromthe readout lobe is used to form a second Dixon acquisition. These firstand second Dixon acquisitions are used in association with a Dixonreconstruction technique to form water and fat images of the scannedobject region.

The system for acquiring the MRI data according to the invention may useany suitable MRI machine capable of constructing an image from the MRIdata sets acquired. Such machines may be MRI machines programmed toexecute a series of instructions for performing the methods describedherein. Such an MRI machine may comprise a field controller forgenerating a magnetic field around a specimen or desired specimen space,a data acquisition mechanism for acquiring the MRI data set using atotal sampling time (TST) acquisition, a processor for determiningrespective pixels in an image being constructed, and other possibleelements such as a frequency parameter representative of a measure ofoff-resonance signals for voxels associated with respective pixels, aprocessor for processing as a function of the frequency parameter, foroff-resonance correction or a processor for performing other imagecorrection techniques. These and other objects of the invention will be,apparent upon reading of the description of embodiments thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood along with its objectsand advantages, with reference to the accompanying drawing.

FIG. 1 is a simplified sequence diagram of a total sampling time (TST)acquisition according to an embodiment of the invention.

FIG. 2 is a flow chart of an embodiment of the method according to theinvention.

FIGS. 3A-C are phantom images, with FIG. 3A being a traditional MRIimage without suppression as a reference, FIG. 3B being a traditional,2-point, 2-acquisition Dixon image, and FIG. 3C being an MRI image usingthe method according to an embodiment of the present invention.

FIGS. 4A-4C are in-vivo head images, with FIG. 4A showing a traditionalimage without suppression as reference, FIG. 4B showing an MRI imageacquired using a traditional 2-point, 2-acquisition Dixon process, and4C being an MRI image formed according to an embodiment of the methodaccording to the invention.

FIG. 5 shows a MRI system for acquiring MRI data according to anembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The systems and methods according to the present invention provide forimprovement over traditional 2-point Dixon techniques for application torectilinear imaging techniques with improved speed and quality. Inconjunction with the fat-water separation provided by Dixon techniques,the present invention provides for increased acquisition speed tofacilitate use in a variety of situations and environments.

Turning now to FIG. 1, a suitable pulse sequence for acquisition of MRIdata from a scanned object region according to an embodiment will bedescribed. In this embodiment, a read gradient of a continuously sampledrectilinear fast low-angle shot (FLASH) pulse sequence is shown with atime between echoes at TE1, TE2 and TE3 set at 2.2 ms providing a ΔTE at2.2 milliseconds at 1.5 T, to take advantage of the chemical shiftbetween water and fat. FIG. 1 shows the read gradient, G_(Read) with thetiming between the start of the applied gradient and the echoes set to2.2 milliseconds in this example. The read gradient includes a dephaseperiod 10, along with a readout period 12 and a rephase period 14. Theuse of total sampling time (TST) acquisition is performed during theentire read gradient, such that data from the dephase lobe 10, andrephase lobe 14 are combined and used as a first Dixon data set, whiledata from the readout lobe 12 is used as a second Dixon data set. It isassumed that both components of water and fat are in the image, and thecombined image at an echo time T_(E,i) is set forth by the followingequation:

M _(i) =m _(ω) +e ^(−iω) _(f) ^(T) _(E,i) m _(f)  Equation 1

where m_(w) is the image of the water component and m_(f) is the imageof the fat component. T_(E,i) is chosen based upon water resonance, soonly the lipid component precesses at ω_(f), set forth as:

ω_(f)T_(E,i)=0, π mod 2π  Equation 2

such that the two resulting images may be set forth as:

m ₁ =m _(ω) +m _(f)  Equation 3

m ₂ =m _(ω) −m _(f)  Equation 4

, which then can be combined to make images containing only water oronly fat, set forth as:

m _(ω)=1 [m _(i) +m ₂]  Equation 5

m _(f)=1 [m ₁ −m ₂]  Equation 6

Optionally, further improvement in image quality may be provided byoff-resonance correction or echo alignment techniques for example.

As an example of an embodiment of the present invention, a pulsesequence to acquire k-space data may be a multislice gradient echosequence, such as a fast low angle shot sequence (FLASH). Such asequence at a magnetic field strength of 1.5 T, may utilize TR valueswhich range from 10-100 ms for example, while utilizing TE values of 3to 50 ms and flip angles of between 10 to 70 degrees as examples. Otherparameters may be used if desired. The pulse sequence may utilize aspoiler gradient after the echo to spoil the steady state by causing aspatially dependent phase shift. This technique spoils the transversesteady state but there is still a longitudinal steady state that dependsupon the T1 values and the flip angle. In accordance with thisembodiment, the TR, TE and flip angle can be manipulated in a desiredmanner to generate suitable image data. TR may be varied, with nominaleffects on contrast as long as the RF pulse is properly chosen. LongerTR's will generally result in higher signal to noise, but with longerscan time. As TR is decreased the optimal flip angle becomes smaller.The TE determines the degree of T2* and sensitivity to motion. The TE isgenerally chosen to be as short as possible. The flip angle is generallychosen to provide desired contrast characteristics. Smaller flip anglesmay be chosen to provide weighted images. Larger flip angles producemore T1 weighting. A 90 degree flip produces a heavily T1 weighted imagebut decreases signal. Other gradient echo sequences may also be usedaccording to the invention, such as true-FISP, FISP, GRASS, SPGR orothers in a similar manner. The gradient echo sequences show a widerange of variations compared to the spin echo and inversion recoverysequences. Not only is the basic sequence varied by adding dephasing orrephasing gradients at the end of the sequence, but there is asignificant extra variable to specify in addition to things like the TRand TE. This variable is the flip or tip angle of the spins. The flipangle is usually at or close to 90 degrees for a spin echo sequence butcommonly varies over a range of about 10 to 80 degrees with gradientecho sequences. For the gradient echo sequence FLASH, the larger tipangles give more T1 weighting to the image and the smaller tip anglegive more T2 or actually T2* weighting to the images.

Using a gradient echo sequence, the spins are generally refocused byreversing the direction of the spins rather than flipping them over tothe other side of the x-y plane as occurs with a spin echo sequence.Gradient refocusing of the spins takes considerably less time than 180degree RF pulse refocusing.

In the example of using a rectilinearly FLASH pulse sequence, as shownin the embodiment of FIG. 1, the sequence utilizes TE's which areselected to allow 180° phase variation in the fat magnetization betweeneach of the three TE's, being TE1, TE2 and TE3 with the time betweenechoes at 2.2 milliseconds. A spoiler gradient is utilized to spoil thetransverse magnetization created by the FLASH magnetization cycle in aknown manner. As is known, the spoiler magnetic field gradient pulse isapplied to effectively remove transverse magnetization by producing arapid variation of its phase along the direction of the gradient. Thisis done after the echo, so that transverse magnetization is destroyedbefore the next excitation pulse, and “spoils” any remainingxy-magnetization or refocuses the xy-magnetization.

The use of FLASH imaging reduces the flip angle and thereby reduces thedependence of the signal on T1 while the use of total sampling time(TST) acquisition allows use of Dixon fat suppression in a singleacquisition. This provides significant improvement over traditionaltwo-point Dixon methods in terms of time savings, while providingrequired suppression of fat or water signals. Accomplishing fatsuppression in one TST acquisition results in significant reductions inacquisition times as compared to traditional two-point Dixon methods oras compared to dual-echo Dixon acquisitions.

In the present invention, the total sampling time acquisition mayutilize a sampling sequence in which sampling is performed during theconstant part of a readout gradient lobe, resulting in equidistantsampling of each k-space area. The invention utilizes additionalacquisition of k-space data in the dephase and rephase portions of thesampling sequence as an example, to thereby extend the acquisitionwindow during the sampling sequence. As will be set forth according toan embodiment of the invention, the acquisition cycle is extended overthe entire positive readout gradient, and further extends theacquisition window to include the negative dephase (or prephaser)portion of the sequence, as well as the rephase portion to allowsampling of additional k-space data. In such an approach, the slopedportions of the primary readout gradient lobe, as well as the slopedportions of the dephase/rephase lobes result in non-equidistant samplingof each k-space phase encode line, which is then accounted for duringimage reconstruction. In this manner, data is acquired during the entiresequence to enable significant shortening of the data acquisition timerelating to the use of Dixon techniques for forming chemical speciessuppression, which in this example may be water/fat separated images.Image reconstruction may use suitable regridding techniques on thek-space data prior to Fourier transform, and additional corrections forreconstruction may be used as desired, such as phase or offsetcorrection.

Turning to FIG. 2, a method according to the invention is depicted,wherein a first step at 20 is to perform a TST acquisition sequence withselected phase variation between multiple TE's. Although the ΔTE of 2.2milliseconds shown in FIG. 1 demonstrates a simple form of the TSTacquisition, this interval is not fixed and other ΔTE's arecontemplated, which could further reduce total acquisition time basedupon the method. At 22, MR data is collected during the dephase gradientlobe, the gradient readout lobe and rephase lobe of the pulse sequence.At 24, the collected MR data during the dephase and rephase gradientlobes is combined to faun a first Dixon acquisition, with MR datacollected during the readout gradient lobe of the pulse sequence forms asecond Dixon acquisition. At 26, algebraic manipulation of the imagedata is performed to form separate water and fat images. At 28,techniques for improving image quality may be optionally performed, suchas off-resonance correction or spin echo alignment techniques.

EXAMPLES

FIGS. 3A-3C and 4A-4C show the results according to use of the methodsof the invention as compared to typical two-point Dixon methods. As aparticular example, a FLASH pulse sequence according to FIG. 1 wasimplemented on a 1.5T Siemens Sonata MR Scanner, with TE1, TE2 and TE3selected to allow 180° phase variation in the fat magnetization betweeneach of the three TE's. Imaging parameters used wereTE1/TE2/TE3=3.4/5.6/7.8 milliseconds, TR=14 milliseconds, FOV=300millimeter², slice thickness=5 millimeters. A second acquisition wascollected with TE2=7.8 milliseconds in order to perform traditional,two-point Dixon fat suppression for comparison. Phantom and clinicalimages were acquired with the TST sequence of FIG. 1, with thedephase/rephase (S_(i)) and readout (S₂) k-space data gridded separatelyusing linear interpolation from a measured trajectory. The water and fatimages were calculated by adding or subtracting S₁ and S₂ and thenperforming 2D Fast Fourier Transform (2DFFT) to image space.Off-resonance correction or other correction techniques can be appliedas desired to the image data.

With respect to the formed images, the contrast-to-noise ratio (CNR) wasmeasured to quantify the level of fat suppression. As an example, CNR iscalculated according to the following equation:

CNR=(I _(water) +I _(fat))/σ_(background),

where I_(water) and I_(fat) are mean signal amplitudes of the water andfat respectively, using a selected region of interest in the image data,and σbackground is the standard deviation of the background signal.

FIG. 3A shows a phantom image of a standard round water phantom and abottle of mineral oil. This figure is a traditional image withoutsuppression and is used as a reference relative to FIGS. 3B and 3C,wherein FIG. 3B shows a phantom image using a typical, two-point,two-acquisition Dixon method. FIG. 3C shows the results of the presentinvention, wherein the continuously sampled, two-point, one-acquisitionDixon phantom image is shown. As is evident from a review of FIGS. 3Band 3C, the use of a TST acquisition sequence provides desired Dixonfat/water separation, while reducing acquisition time approximately inhalf. In calculating CNR for these images, the CNR for the water imagesshown in FIGS. 3B and 3C were 8.6 and 8.7 respectively. Similarly, asshown in FIGS. 4A-4C, an in-vivo head image is shown with FIG. 4Ashowing the traditional image without suppression as referenced. Atraditional, two-point, two-acquisition Dixon image is shown in FIG. 4B,while the continuously sampled, two-point, one-acquisition Dixon imageaccording to the method of the present invention is shown in FIG. 4C. Incalculating CNR for the water images produced were 30.4 and 28.0 forFIGS. 4B and 4C respectively. In the images shown in FIGS. 3 and 4, theimages are not corrected for off-resonance as for these particularcases, images before and after correction were virtually identical.

The use of a TST acquisition provides Dixon fat suppression in a singleacquisition to substantially reduce acquisition time. The results of thesystems and methods show nearly identical suppression of fat or water ascompared to typical 2 point Dixon methods. Selection of reduced ΔTE'smay also provide reduction of total acquisition times while achievingdesired suppression of fat or water.

FIG. 5 illustrates one example of a magnetic resonance system 90 of thetype that can be used with the systems and methods described above.Other MRI systems are known to those skilled in the art, and arecontemplated according to the invention. The system 90 includes a basicfield magnet 92 supplied by a basic field magnet supply 94. The systemhas gradient coils 96 for respectively generating gradient magneticfields G_(s), G_(p) and G_(r), operated by gradient coil supply 98. AnRF antenna 100 is provided for generating RF pulses, and for receivingresulting magnetic resonance signals from an object being imaged. The RFantenna 100 is operated by an RF transmission/reception unit 102. The RFantenna 100 may be employed for transmitting and receiving, oralternatively, separate coils may be provided for transmitting andreceiving if desired. The gradient coil supply 98 and the RFtransmission/reception unit 102 may be operated by a control system 104to selectively produce radio frequency signal pulses directed to theobject to be imaged, and to generate gradient magnetic fields forlocalizing a region of resonance. The magnetic resonance signalsreceived by the RF antenna are processed into image data, such as by 2DFast Fourier Transform (2DFFT) such as by an image computer 108 or othersuitable processing system. The image data may then be shown on adisplay 109. According to the present invention, the MRI apparatus isoperated and acquires MR data according to the TST acquisition asdescribed.

Although various embodiments of the present invention have been shownand described herein, various changes and modifications would occur tothose skilled in the art without departing from the scope of theinvention. Therefore, the appending claims encompass within their scopeall such changes and modifications, and are not to be limited thereby.

1. An apparatus, comprising: an MRI scanner for obtaining images of apredetermined volume; a controller configured to control operation ofthe scanner to acquire images using a modified Dixon technique wheredata acquisition for the modified Dixon technique is performed over areadout lobe and one or more of, a dephase lobe, and a rephase lobe of agradient echo pulse sequence performed by the scanner; where dataacquired during one or more of the dephase and rephase lobes form afirst Dixon MRI data set and data acquired during the readout lobe isused to form a second Dixon MRI data set; a processing system to processthe first Dixon MRI data set and the second Dixon MRI data set using aDixon reconstruction technique to create a first chemical species imagedata set and a second chemical species image data set, and an outputdevice for displaying a first chemical species image and a secondchemical species image, based, at least in part, on the first and secondchemical species image data sets.
 2. The apparatus of claim 1, where thegradient echo pulse sequence is selected from the group consisting of aFLASH sequence, a true-FISP sequence, a FISP sequence, a GRASS sequence,and a SPGR.
 3. The apparatus of claim 1, where the gradient echo pulsesequence uses TR values ranging from 10-100 ms, TE values ranging from 3to 50 ms, and flip angles of between 10 to 70 degrees.
 4. The apparatusof claim 3, where the TE values are chosen to allow 180 degree phasevariation between the first and second chemical species magnetizationbetween each TE.
 5. The apparatus of claim 1, where the MRI scanneracquires k-space data at a first TE, a second TE, and a third TE toacquire signal components from a first chemical species and from asecond chemical species at the TE times.
 6. The apparatus of claim 1,where the processing system performs an off resonance correction methodto suppress the effects of local magnetic field inhomogeneity on thefirst chemical species image data set.
 7. The apparatus of claim 1,where the processing system performs an off resonance correction methodto suppress the effects of local magnetic field inhomogeneity on thesecond chemical species image data set.
 8. The apparatus of claim 1,where the first chemical species is water and the second chemicalspecies is fat.
 9. The apparatus of claim 1, where the first and secondDixon MRI data sets are processed using a 2-point Dixon reconstructiontechnique.
 10. A method, comprising: acquiring k-space data over theread gradient and one or more of a dephase lobe and a rephase lobe of agradient echo pulse sequence; forming a first Dixon acquisition fromk-space data from one or more of the dephase and rephase lobes of thegradient echo pulse sequence and forming a second Dixon acquisition fromk-space data from the readout lobe; and forming first and secondchemical species images from the first and second Dixon acquisitionsusing a Dixon reconstruction technique.
 11. The method of claim 10,where the gradient echo pulse sequence is selected from the groupconsisting of a FLASH sequence, a true-FISP sequence, a FISP sequence, aGRASS sequence and a SPGR.
 12. The method of claim 10, where thegradient echo pulse sequence uses TR values ranging from 10-100 ms, TEvalues ranging from 3 to 50 ms, and flip angles of between 10 to 70degrees.
 13. The method of claim 11, where the FLASH sequence uses TRvalues ranging from 10-100 ms, TE values ranging from 3 to 50 ms, andflip angles of between 10 to 70 degrees.
 14. The method of claim 13,where the TE values are chosen to allow 180 degree phase variationbetween the first and second chemical species magnetization between eachTE.
 15. The method of claim 10, where the gradient echo pulse sequenceis a FLASH sequence using three TE values in the pulse sequence, with atime between echoes selected to be 2.2 milliseconds, for purposes ofperforming two-point Dixon processing, where each TE is a multiple of2.2 milliseconds.
 16. The method of claim 15, where three TE values areused in the pulse sequence comprising a first TE of 3.4 milliseconds, asecond TE of 5.6 milliseconds, and a third TE of 7.8 milliseconds. 17.The method of claim 11, where acquiring k-space data is performed by theMRI scanner at a first TE, a second TE, and a third TE to acquire signalcomponents from both the first and second chemical species at each ofthe TE times.
 18. The method of claim 11, comprising: performing an offresonance correction method to suppress the effects of local magneticfield inhomogeneity on the first chemical species image data set. 19.The method of claim 11, comprising: performing an off resonancecorrection method to suppress the effects of local magnetic fieldinhomogeneity on the second chemical species image data set.
 20. Anapparatus, comprising: a MRI scanner for obtaining images of apredetermined volume; a controller configured to control operation ofthe scanner to acquire images using a modified Dixon technique wheredata acquisition is performed over a read gradient and one of a dephasepulse and a rephase pulse of a gradient echo pulse sequence performed bythe scanner to produce a plurality of magnetic resonance imaging datasignals; a processing system to process the plurality of MRI datasignals using a Dixon reconstruction technique to create a firstchemical species image data set and a second chemical species image dataset, and an output device for display of a first chemical species imageand a second chemical species image; where the gradient echo pulsesequence uses TR values ranging from 10-100 ms, TE values ranging from 3to 50 ms, and flip angles of between 10 to 70 degrees; where the TEvalues are chosen to allow 180 degree phase variation between the firstand second chemical species magnetization between each TE; and where aFLASH sequence is used with three TE values used in the pulse sequence,with a time between echoes selected to be 2.2 milliseconds, for purposesof performing two-point Dixon processing, where each TE is a multiple of2.2 milliseconds.
 21. An apparatus, comprising: an MRI scanner forobtaining images of a predetermined volume; a controller configured tocontrol operation of the scanner to acquire images using a modifiedDixon technique where data acquisition is performed over a read gradientand one or more of a dephase pulse and rephase pulse of a gradient echopulse sequence performed by the scanner to produce a plurality ofmagnetic resonance imaging data signals; a processing system to processthe plurality of MRI data signals using a Dixon reconstruction techniqueto create a first chemical species image data set and a second chemicalspecies image data set, and an output device for display of a firstchemical species image and a second chemical species image; where thegradient echo pulse sequence uses TR values ranging from 10-100 ms, TEvalues ranging from 3 to 50 ms, and flip angles of between 10 to 70degrees; where the TE values are chosen to allow 180 degree phasevariation between the first and second chemical species magnetizationbetween each TE; where a FLASH sequence is used with three TE valuesused in the pulse sequence, with a time between echoes selected to be2.2 milliseconds, for purposes of performing two-point Dixon processing,where each TE is a multiple of 2.2 milliseconds; and where three TEvalues comprise a first TE of 3.4 milliseconds, a second TE of 5.6milliseconds, and a third TE of 7.8 milliseconds is used in the pulsesequence.